Site‐Selective Ligand Functionalization Reverses Hypsochromic Luminescence Shifts in Platinum(II) Complexes of Benzannulated <i>NCN</i>‐Coordinating Ligands

Ligands containing phenanthridine (benzo[c]quinoline) have presented notable exceptions to the conventional logic that increasing ligand benzannulation leads to bathochromic (red) shifts in the absorption and emission of their coordination complexes. The counterintuitive blue shifts have been attributed to the peculiar structure of phenanthridines, whose ground states are dominated by imine-bridged biphenyl resonance contributors. These serve to isolate the C=N unit electronically from the rest of the ligand framework and allow the C=N moiety to act as a 'shock-absorber', buffering against larger molecular distortions in a molecule's excited state, and reducing the observed pseudo-Stokes' shift. Here, we provide experimental evidence for this assertion in the form of a counterfactual that reverses this trend: substitution at the phenanthridine 6-position (i. e., at the C=N sub-unit) breaks the phenanthridine's tendency to cause hypsochromic luminescence shifts. The synthesis, full characterization, and comparison of 2-quinolinyl and 6-phenanthridinyl exemplars is provided, supported by a detailed theoretical treatment.